Help - How do I remove an arching red gradient at top an image ?

[RobertM]

Don't know how this appeared but any ideas about removel would be most welcome.



Many thanks
Robert

[Rick]

My guess is you're seeing a difference in sensitivity at different wavelengths, and may need a flat-field taken using the same filter to compensate?

Or it might just be that you need a different dark-frame if the red was using a different exposure time?

...but I'm just guessing.

There are also some grey-green-red tripples which may be the result of hot pixels?

Looks promising, though.

[Rocket Pooch]

Hi Robert,

If you software you use had a measurement profile tool you should be able to click from the top of the RED frame to the bottom and see how many ADU's extra the gradient is.  Then you should be able to use a Gradient correction tool to subtract the ADU's from the top of the Frame.

I use AIP and the tools are called Measure->Profile and File->Gradient removal, I don;t know what it is in AA or Maxim, but they will have these features.

Do this before combining the RGB then should should be able to remove most of it.

The other way is to create a false flat in photoshop and then remove the flat from the sub, see here for details.

http://www.deep-sky-photography.com/M27-HaO3-RGB-HX916.html

Chris

[RobertM]

Brilliant, that's just what I needed ( and something to aspire to in the bargain ! ) - will try tonight if I get time.

I did loose a grub screw from the NGF-S, where it screws in to the reducer, so it could be that it's leaning a bit with all the weight and letting in some light.  It's just a theory but the gradient is only there on the red channel which isn't far off the spectral peak of those low pressure sodium lights that illuminate the telescope.

Many thanks again.